Do you have measurements or is this just something you are guessing?
There are photos showing the height of the light and how it is over the table. Since it was over the table it had to be hit standing to the side of the table and hit when the gun was at an angle and not while walking underneath the light.
In a thread where I discussed this in the past someone posted a photo of the light

and I posted the following pic of someone (some claim it is AP) holding the gun with the moderator attached. The measurement of the gun sans moderator is only 3.5 feet (109CM). Sheila was only 5'7" so even if the gun extended a foot above her head that would not be good enough to result in the light being hit.
While arguing over the gun she would be holding it at chest level because that provides the most leverage. It would be easy to take away if held eye level. Even with the moderator she would have to lift the rifle up in an unnatural position to hit the light because of her height. Her father and brother were over 6 feet. Them wrestling over the gun with the moderator attached would allow the shade to be hit because they were taller and at their chest level with the moderator attached it would be able to reach the bottom of the lampshade. That is if the gun were held like this guy is holding it as they wrested as opposed to horizontal. If horizontal obviously the light still would not be hit.
Jeremy was sly asking how the lampshade broke, he wanted to know if they figured out Neville and his killer wrested for control of the weapon and/or to make it appear he was perplexed as to how it could have gotten broken to help fool them about him not being the perp.

The scratching of the mantle indicates that as they wrestled the gun was twisted down and that enabled it to scrape the mantle as they struggled for control. So the gun was in the opposite position of the photo and diagonally down at some point. It must have been some struggle that the gun was going all over. Neville did indeed put up a good struggle as Jeremy old Julie. How did Jeremy know he put up such a good struggle unless he were the killer?